A process for producing a liquid crystal display device (LCD) generally comprises a step of forming an array on a glass substrate, a step of forming a color filter on a glass substrate different from the above glass substrate, a step of bonding the glass substrate having an array formed thereon with the glass substrate having a color filter formed thereon (an array/color filter bonding step), a step of injecting liquid crystal and a step of sealing the injection inlet. In each of the steps, since a back surface of the glass substrate, namely, an opposite surface from the surface on which an array or a color film is to be formed, is directly contacted with a carrier jig, a hotplate, etc., whereby microscopic scars are formed on its surface, which may cause a strength decrease of the glass substrate itself.
In the field of a medium or small sized liquid crystal display device (LCD), an organic EL display device (OLED), particularly, a portable display device such as a mobile, a digital camera or a cellular phone, weight saving and reduction in thickness of a display device have been important objects, and since the reduction in thickness of the glass substrate has been progressed, the strength decrease of the glass substrate originated from such a step may especially be a serious problem.
Further, in order to make a glass substrate thinner, a step of carrying out a chemical etching treatment on a glass substrate to make the substrate thickness thin after the array/color filter bonding step, has been widely applied. However, in a case where if microscopic scars formed in the above step are present on the glass substrate, pits (etch pits) having a diameter of from tens to hundreds μm will be formed on the glass substrate surface after the chemical etching treatment, which would lead to optical defects.
Therefore, in order to overcome the above problem, a process for producing a display device has been suggested, wherein a prescribed treatment for producing a display device is carried out in such a state that a glass substrate is bonded with another protective glass substrate, and after the completion of the treatment, the glass substrate and the protective glass substrate are separated (Patent Documents 1 to 6).
In these processes for producing a display device, a method for laminating and fixing the glass substrate and the protective glass substrate on each other may, for example, be a method of fixing them by means of electrostatic adsorptivity or vacuum adsorptivity caused between the glass substrates (e.g. Patent Document 1), a method of fixing both ends of the glass substrates by means of glass frit (e.g. Patent Document 2), a method of fusing the two glass substrates by irradiating the vicinity of the edge surfaces of peripheral portions with laser beams (e.g. Patent Document 3), and a method of disposing a removable adhesive or adhesive sheet over the entire surfaces of the glass substrates, and fixing them by the adhesive force (e.g. Patent Documents 4 to 6).
These methods have latent problems which may adversely affect a display device to be produced.
That is, by the method of fixing the glass substrates by means of electrostatic adsorptivity or vacuum adsorptivity, the method of fixing both ends of the glass substrates by means of glass frit, or the method of fusing the two glass substrates by irradiating the vicinity of edge surfaces of peripheral portions with laser beams, it is difficult to avoid inclusion of bubbles or convex defects due to foreign matters such as dust in the process of laminating and closely bonding the glass substrates without any interlayer, and accordingly it is difficult to obtain a glass substrate laminate having a smooth surface.
In the case of the method of disposing a removable adhesive or adhesive sheet over the entire surface of the glass substrates, it is easy to avoid inclusion of bubbles as compared with the case of directly laminating the glass substrates, and it is considered that convex defects due to foreign matters are less likely to occur. However, it is difficult to separate the glass substrate and the protective glass substrate, and the glass substrate may be broken at the time of separation. Further, remaining of the adhesive on the glass substrate after the separation is also problematic. Further, the display device production process comprises a step which requires treatment at high temperature, such as a step of firing an insulating film or an alignment film in a process for producing a liquid crystal display device. Therefore, heat resistance is required for the adhesive or adhesive sheet, to be used for the display device, but a method which satisfies both heat resistance and removability has not been proposed yet.    Patent Document 1: JP-A-2000-241804    Patent Document 2: JP-A-58-54316    Patent Document 3: JP-A-2003-216068    Patent Document 4: JP-A-8-86993    Patent Document 5: JP-A-9-105896    Patent Document 6: JP-A-2000-252342